The calcium-phospholipid-phosphate complex isolated from young rabbit bones has been shown to be comparable to an intermediate formed prior to the appearance of hydroxyapatite when the acidic phospholipids phosphatidyl serine and phosphatidyl inositol are placed in metastable calcium-phosphate solutions. In order to find out whether the inorganic phosphate is an essential part of both the synthetic and bone isolated complexes the kinetics of hydroxyapatite proliferation by synthetic Ca-phospholipid and Ca-phospholipid-PO4 complexes will be compared. The synthetic and bone isolated complexes will be examined by Infra-red spectroscopy and wide angle x-ray diffraction to determine respectively: the phospholipid moieties with which the Ca is associated; and the effect of Ca and PO4 as opposed to Ca alone in the lamellar spacings of the phospholipid. In order to confirm the finding (based on studies of rabbit long bones) that the proportion of total lipid that is complexed to Ca varies inversely with age, a series of calcified tissues will be examined as a function of age. These tissues will include metaphyseal and diaphyseal bone, skull, scapula, dentin, and epiphyseal and articular cartilage obtained from rabbits ranging in age from fetal animals to 2 years. BIBLIOGRAPHIC REFERENCES: A. L. Boskey and A. S. Posner, Formation of Hydroxyapatite at Low Supersaturation. J. Phys. Chem., 80, 40-45, 1976. A. L. Boskey and A. S. Posner, Extraction of a Calcium-Phospholipid-Phosphate Complex from Bone. Calc. Tiss. Res., 19, 273-283, 1976.